Carbon nanotubes are allotropes of carbon containing one or more layers of graphene rolled into a cylindrical shape. Although carbon nanotubes can have lengths of up to several millimeters, their diameters are on the nanoscale, e.g., from less than one nanometer to about 50 nanometers. Carbon nanotubes can exhibit important mechanical, electrical, and thermal properties, and therefore can have great potential in a variety of applications.
The specific structure of a carbon nanotube can determine its properties. For example, carbon nanotubes can be semiconducting or metallic, depending on the chiral angle of the graphene. Semiconducting carbon nanotubes and metallic carbon nanotubes can be useful for different applications, and thus it can be desirable to isolate carbon nanotubes based on these electrical properties. Such isolated carbon nanotubes can find use in, for example, supercapacitors, ion battery electrodes, fuel cells, solar cells, lightweight electromagnetic shielding, sensors, and transistors.
Certain methods of separating semiconducting carbon nanotubes and metallic carbon nanotubes are known in art. For example, gel electrophoresis or density gradient ultracentrifugation have been used to separate carbon nanotubes. However, these methods can be cumbersome and expensive. U.S. Pat. No. 7,161,107 describes a method of separating carbon nanotubes by performing dielectrophoresis on a solution containing carbon nanotubes and a solvent. The method applies a non-homogeneous electric alternating field to the solution to cause the carbon nanotubes to migrate and separate.
The dielectrophoretic force (FDEP) that a particle experiences when dispersed in a solution is approximately proportional to the gradient of the applied electric field intensity profile (E2({right arrow over (r)})) multiplied by the Clausius Mossotti Function (CMF), as represented by Formula 1:FDEP({right arrow over (r)})∝[CMF]∇E2({right arrow over (r)})  (Formula 1)
The high frequency limit gives a dominating term of the Clausius Mossotti Function, and is approximated by Formula 2.
                                          lim                          ω              ->              ∞                                ⁢                                  ⁡                          [              CMF              ]                                      ->                                            ϵ              p                        -                          ϵ              l                                            ϵ            l                                              (                  Formula          ⁢                                          ⁢          2                )            
In Formula 2, ω represents frequency, ∈p represents particle permittivity, and ∈l represents liquid permittivity. Thus, applying a non-homogeneous electric alternating field to a solution containing semiconducting carbon nanotubes and metallic carbon nanotubes, having particle permittivities ∈p(s) and ∈p(m), respectively, can separate the semiconducting carbon nanotubes and the metallic carbon nanotubes as long as one particle permittivity is less than the liquid permittivity and the other particle permittivity is greater than the liquid permittivity, as represented in Formula 3.∈p(s)<∈l<∈p(m)  (Formula 3)
In certain methods, the electric field is provided as an electromagnetic wave. However, due to its transience, only a small portion of the energy in the wave goes to separating the carbon nanotubes. Therefore, although such methods can separate semiconducting carbon nanotubes and metallic carbon nanotubes, it is accomplished at a loss of power.
Thus, there remains a need in the art for improved techniques for separating carbon nanotubes.